[b]"I've got a buddy convinced that potatos are the only food for long endurance activities because of the starch. He states that the US army, in some crazy year like 1946, decided that the potato was the ultimate endurance food."[/b]
Could be... but that's not the rate-limiting step anyway... endurance is multi-faceted and it's only partially determined by calorie intake... let's back up a bit... I'll break up this discussion on endurance into two related systems, energy and muscular.
First, let's examine our energy stores in relation to endurance:
2000~2500 calories in muscle-glycogen
500~750 calories glycogen in liver and floating around bloodstream
400~600 calories in free proteins and amino acides in bloodstream
unlimited calories in fat-stores
Obviously the main limitation is the 2500 calories in our muscles as that's accessed first and its supply must be replenished from outside sources. The question here is a matter of HOW best to re-stock the muscle-glycogen stores.
Personally, I hate using the glycemic index as a proxy indicator of blood-glucose levels, and it's also not a good indicator of muscle-glycogen stores either. However, measuring insulin is easier than glucose concentrations but it's still a secondary indicator, kinda like using the cholesterol marker to track relative concentrations of HDL to LDL fats. But it's the best we've got to work with, so let's make the most we can of the glycemic index and its relation to measuring endurance factors.
Now the main rate-limiting step in endurance is getting the CHO molecules past intestinal lining into the bloodstream where it can be transported to the muscles to replace the spent muscular glycogen. The age-old idea of using slow-release carbs like complex starches that takes a long time to digest may be fine for dieters and people who don't want to drive up their glycemic-index and the resultant fat-storage, but for athletes, it's doesn't matter WHEN all the CHO gets through, it's a matter of HOW MUCH we can get RIGHT NOW!!!. The trick is getting the highest calories/hour of CHO into the bloodstream.
It does appear that training and an optimized nutrition mixture does affect the rate of CHO absorption. TDF and RAAM teams have seem to fine-tuned this process to about 400 cal/hr on the upper limit with 300-350 cal/hr to be easily obtainable, much higher than the 200-250 cal/hr rate for mere mortals like us. Now if we assume that the glycemic-index is an indicator of absorption-rate of various carbs, then yes we want a high GI CHO in order to get it from the intestines into the bloodstream as fast as possible. It appears medium-chain carbs work best, too big of a molecule takes too long to digest before its absorbed, to simple, like glucose, ends up being too dilute a mixture to get through a lot at once. The potato's high GI seem to indicate that it's a good balance in the middle and is a fast-absorbing CHO, good for performance.
You also want to avoid fructose like the plague, so that leaves out dried fruits, fruit-juices and sodas sweetened with high-fructose corn-syrup (that's an evil requiring another dissertation altogether). On reason the GI isn't affected by fructose is that fructose is useless to the body. It cannot be converted to glycogen and cannot be used in ATP-production by the muscles, it cannot be converted to fat either. All fructose must be metabolised in the liver first and coverted into glucose and returned to the bloodstream before it can be used, not like the liver doesn't have enough to do already... so avoid fructose.
The insulin-response to the absorbed CHO on the other hand, doesn't appear to affect endurance in any way. Typically insulin is to signal body cells to absorb free-floating blood-glucose into muscle-tissue glycogen. When those are well-stocked, it also triggers the conversion to fat in adipose tissues as well. However, when you're running a glycogen defecit in the muscle tissues, regardless of insulin levels, the digested CHO is headed straight for your muscles anyway. Experiments, primarily on race-horses, show that pre-race meals and their resultant insulin levels doesn't affect performance whether or not you're well-fed. What matters in endurance is replacing all the glycogen that you burn up later.
Another factor that affect CHO absorption aside from GI is concentration in the stomach. The RAAM teams have found it's best to keep CHO concentrations high and limit the water used. This is to get as much carbs past the stomach into the intestines and too much water dilutes the mix and limits how many CHO molecules makes in in each "squirt" out of the stomach. The RAAM riders that end up dehydrated at the end of the day seems to have better endurance than those who take in water 1:1 with their expenditures. Plus too much water tends to cause gastrointestinal cramps and other distractions from performance.
Another factor is adequate electrolyte consumption along with the CHO. Seems about 1000mb of sodium salt per hour is what the RAAM teams use and it balances out what's lost from perspiration. Magnesium and potassium salts are also vital for managing smooth muscle contractions and avoiding cramps. Again, another area where the potato comes in handy.
One little known factoid in maintaining electrolytes is in absorption of CHO into the intestions. Each and every single glucose molecule that's absorbed through the intestine requires an exchange with a sodium. With low sodium levels, you can't absorb the carbs you're consuming, a vicious cycle when you're bonking, eh? You're going crazy and desperately looking around on the ground for scraps of carbs to eat and the body's refusing to absorb it... heh, heh 🙁
So, the nutrition side to endurance is easy. While getting 300 cal/hr may not be able to completely supply a 200-mile ride, but I've found that there's some other factors other than the energy system that help made those rides easier. One of them, the muscular system, really impacts endurance and improved my average speed on a 200-mile ride... more on that in another paper.. 🙂
